The invention relates to an auditive prosthesis comprising an elongated carrier which can be implanted in the scala tympani of a cochlea, which carrier is provided, over at least a part of its length, with electrode elements at a first side face and, at a second side face facing away from the first side face, with an electric conductor, which side faces are interconnected by a third side face and a fourth side face, the first side face extending, in a state in which the carrier is implanted in the scala tympani, opposite the basilar membrane of the cochlea.
Such an auditive prosthesis is known from WO 97/38653 (PHQ 96.025). The known prosthesis comprises a silicone carrier provided with a set of electrodes, which silicone carrier can be operatively implanted in a scala tympani of a cochlea to improve the hearing capacity of a hearing-impaired person. Said set includes a series of electrode contacts arranged on a side of the carrier and a central electrode arranged on an opposite side. After implantation of the carrier, electric currents can be sent through the electrodes, in dependence upon received sound signals, to stimulate nerve fibers of the auditory nerve present in the modiolis so as to induce electrical signals into the auditory nerve. Via nerve fibers, these signals are guided to the brain to interpret sound signals.
Said known prosthesis is provided with a swelling member which is provided on a side face interconnecting the two above-mentioned sides, which swelling member, after implantation of the carrier, serves to urge the carrier against the modiolis. Although a reasonable stimulation of the auditory nerve can be achieved in this way, relatively many electric currents between the electrode contacts and the central electrode still run via the perilymph, so that considerable parts of the electric currents supplied for stimulation are lost and hence only a limited information transfer to the neural system in the modiolis takes place.
It is an object of the invention to improve the auditive prosthesis mentioned in the opening paragraph in such a manner that, after implantation, said prosthesis can highly efficiently transfer information to the auditory nerve.
To achieve this, the auditive prosthesis in accordance with the invention is characterized in that, over at least a part of the length of the carrier where electrode elements are present, the third side face and the fourth side face of the carrier are situated at distances from each other which are at least substantially equal to corresponding dimensions of the scala tympani which extend parallel to the basilar membrane, in order to at least substantially completely fill the scala tympani, after implantation, over at least a part of the area extending between the fenestra cochlea (round window) and the helicotrema in a zone extending parallel to the basilar membrane.
The auditive prosthesis in accordance with the invention is intended to bring about a sound perception in patients who are hard of hearing, very hard of hearing or deaf, which sound perception may give rise to the hearing of speech by means of an efficient electrical stimulation of the auditory nerve.
The above-mentioned carrier with electrode elements forms an internally implantable part of the auditive prosthesis in accordance with the invention. The auditive prosthesis may additionally comprise an externally implantable part, which is provided with means for receiving sound signals and with a speech processor for converting these signals to electrical signals. By means of algorithms, the sound caught can be processed to imitate the operation of a normal inner ear. Such a processing step may result in coded information which can be passed on to the internal part where this information is converted to electric currents between the electrode elements and the conductor present. The number of individually drivable electrode elements may range, for example between 28 and 35, although other numbers are possible too.
By means of the measure used in the auditive prosthesis it has been achieved that, after a correctly performed implantation, the first side face of the carrier faces the basilar membrane, the electrode elements being very close to the habenula perforations. The habenula perforations are openings in the bone structure bordering on the scala tympani, which openings guide the dendrites of the afferent nerve fibers of the internal hair cells along the ganglion cells to the higher auditive centers in the brain. The applied measure leads to a well-definable interface between the carrier and the neural system, which can be attributed to the fact that the space of the scala tympani can be filled in such a way by a correctly implanted carrier that in the zone extending parallel to the basilar membrane there is no, or hardly any perilymph between the carrier and the partition of the scala tympani, so that the resistance path between the electrode elements at the first side face and the conductor at the second side face is maximal. Consequently, the currents generated for stimulation are more efficiently used to stimulate the nerve fiber structure in the modiolis.
An embodiment of the auditive prosthesis in accordance with the invention is characterized in that, over at least the part of the carrier where electrode elements are present, the first side face and the second side face of the carrier are at distances from each other which are at least substantially equal to corresponding dimensions of the scala tympani which extend transversely to the basilar membrane, in order to at least substantially completely fill the scala tympani, after implantation, over at least a part of the area extending between the fenestra cochlea and the helicotrema in a zone extending transversely to the basilar membrane.
By means of the measure applied in this embodiment it is achieved, after a correct implantation of the carrier in the scala tympani, that the first face of the carrier, which is provided with electrode elements, lies against the basilar membrane and the adjoining part of the modiolis, in particular the laminae spiralis osseae, which has a favorable effect on the transfer of electric currents from the electrode elements to the habenula perforations. Besides, electrode elements situated as closely to the nerve fibers as possible counteract disturbing channel interactions.
An embodiment of the auditive prosthesis in accordance with the invention is characterized as defined in claim 3.
An embodiment of the auditive prosthesis in accordance with the invention is characterized as defined in claim 4.
In the practical embodiments in accordance with claims 3 and 4, the carrier is optimally adapted to the anatomy of the cochlea of human beings.
In an embodiment of the auditive prosthesis in accordance with the invention the first side face of the carrier is an at least substantially flat or, possibly, slightly curved surface, the electrode elements being provided with flat contact faces which are situated in or below the above-mentioned surface.
Consequently, the first side face does not have projecting parts, so that said face accurately corresponds to the shape of the inner side of the scala tympani which is situated near the basilar membrane and the adjoining part of the modiolis, which, on the one hand, is important to preclude damage to the basilar membrane during the provision of the carrier and, on the other hand, has a favorable effect on the information transfer, after the provision of the carrier. If the contact faces do not extend in the surface of the carrier, they are preferably situated directly underneath said surface. This is the case, for example, if the electrode elements are retained in the carrier by means of an overhanging edge of carrier material.
It is noted that the application of the measure in accordance with claim 1, whether or not in combination with the measure in accordance with claim 2, leads to a carrier which is relatively voluminous of itself and hence to an auditive prosthesis of which at least two side faces, even four side faces if the measure in accordance with claim 2 is applied, may closely contact the partitions of the scala tympani during inserting the carrier into the scala tympani. It has nevertheless been found that this does not have any adverse effects for the patient if the implantation is carried out correctly and if a flat first side face is used. In addition, it has surprisingly been found that the prosthesis with a flat first side face can be removed from the cochlea without damaging the scala tympani, more specifically the basilar membrane.
An embodiment of the auditive prosthesis is characterized in that the contact faces of the electrode elements are arranged according to a series extending in the longitudinal direction of the carrier, at least a number of the contact faces present being oval faces whose longitudinal axes are oriented transversely to the longitudinal direction of the carrier. An advantage of this embodiment resides in that, in spite of a high density of electrode elements, the carrier has a relatively high flexibility, which is favorable during inserting the carrier into the spiral-shaped scala tympani. The applied measure also enables the use of relatively large contact faces, which has a favorable effect on the impedance between the contact faces and the perilymph. In addition, the use of larger contact faces enables a smaller current density to be used, which is important in precluding undesirable electrochemical reactions at the location of the contact faces.
An embodiment of the auditive prosthesis in accordance with the invention is characterized in that the contact faces of the electrode elements have a rough surface. Contact faces having a certain degree of roughness have a larger effective contact surface area and hence a lower impedance than smooth contact faces. Preferably, the contact faces of the auditive prosthesis are provided with a coating, in particular an impedance-reducing coating, such as iridium oxide, in which case the roughness also has a favorable effect on the adhesion of the coating. The electrode elements themselves are preferably made of an inert material, such as platinum.
The invention further relates to a carrier which can suitably be used in the auditive prosthesis in accordance with the invention. The carrier in accordance with the invention is characterized as described in any one of the claims 1 through 7.
The invention further relates to an elongated, preferably pre-bent, carrier which is provided over at least a part of its length, at a first side face, with electrode elements and which can be implanted in one of the tubular spaces, such as the scala tympani, the scala vestibuli or the scala media, of a cochlea. In order to enable the side face provided with electrode elements to connect closely, after the implantation of the carrier, to a corresponding wall part of the cochlea, the carrier in accordance with the invention is characterized in that the first side face is an at least substantially flat or, possibly, slightly curved surface, the electrode elements being provided with flat contact faces situated in or below said surface. Consequently, in said first side face there are no projecting parts, thereby counteracting damage of the cochlea during implantation, and enabling, after implantation, said side face to be in close contact with an inner wall of the cochlea. The carrier in accordance with the invention thereby achieves the object that it should be possible, after implantation, to highly efficiently transfer information to the auditory nerve.
Preferably, the contact faces of the electrode elements are arranged in accordance with a series extending in the longitudinal direction of the carrier, at least a number of the contact faces present being oval or oval-like faces having axes which are oriented transversely to the longitudinal direction of the carrier. The carrier will generally be provided at a second side face with one or more electric conductors to form a suitable electric circuit.
In an embodiment of the carrier in accordance with the invention, the contact faces of the electrode elements have a rough surface.
With respect to the claims it is noted that various combinations of characteristics defined in the claims are possible.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.